1. Field of the Invention
The present invention relates to an image reading element used in an image sensor for converting an image signal to an electric signal and outputting the converted electric signal. More particularly, the present invention relates to a sensor using an organic photoconductive material as a photoelectric converting material.
2. Description of the Related Art
An optical sensor is widely used as a position sensor in measurement of light intensity, a robot and various kinds of automatic systems. The optical sensor is also widely used in reading of image information in information communication, information processing, etc. In particular, since techniques and abilities of image information processing are advanced at present, it is very desirable to develop an image sensor as an input device of image information having high performance. The image input device is typically required in a facsimile telegraph, a word processor, an electronic filing system, etc.
Such an input device is divided into a device for outputting two-dimensional information of a video camera, etc. and an image scanner for scanning and reading an image by using a line sensor. The image scanner using the line sensor is normally used to obtain sufficient resolving power and a sufficient number of pixels. The line sensor is typically constructed by a charge coupled device using crystal silicon. However, there is a limit in size of the charge coupled device. Accordingly, when an image having a large area is read, it is necessary to use an optical system for reduction or arrange many charge coupled devices with high accuracy. In contrast to this, in the case of a sensor having cadmium sulfide and amorphous silicon as a photoconductive face, a size of this sensor can be set to be relatively large and a line sensor of an equal magnification contact type is partially used practically by using a rod lens array.
However, there is a limit in a film forming method with respect to such a general photoconductive material so that mass productivity is low. Accordingly, it is substantially difficult to manufacture a long line sensor for scanning an image having a large area. In contrast to this, in the case of a sensor using an organic material as a photoconductive material, a film can be easily formed by coating of coating liquid. Accordingly, productivity of this sensor is excellent and this sensor is easily large-sized. Further, dark conductivity of this sensor is low and a signal-to-noise ratio can be set to be large. For example, Japanese Patent Application Laying Open (KOKAI) Nos. 61-285262, 61-291657 and 1-184961 show some known examples of an image sensor using an organic material as a photoconductive layer.
However, in such an image sensor using the organic photoconductive material, resolving power is low in comparison with an image sensor using an inorganic photoconductive material so that it is difficult to obtain a preferable output image. In the following description, this resolving power is called MTF for short.
MTF is a value defined by the following formula, EQU [(V.sub.max -V.sub.min)/(V.sub.max +V.sub.min)].times.100(%)
where V.sub.max is a maximum value of an output voltage of the image sensor and V.sub.min is a minimum value of this output voltage. MTF can be measured by inputting an original having white and black portions arranged at an equal distance to the image sensor.
When the resolving power (MTF) is practically equal to or greater than 30%, the original can be read with relatively high resolution so that an excellent output image can be obtained.